mod+internal: copy MuSig2 v0.4.0 code from btcd/btcec/v2

With this commit we copy the exact code of the MuSig2 code as found in
github.com/btcsuite/btcec/v2/schnorr/musig2 at the tag btcec/v2.2.2.
This corresponds to the MuSig2 BIP specification version of v0.4.0.

.golangci.yml

@@ -2,11 +2,13 @@ run:
   # timeout for analysis
   deadline: 10m
 
-  # Skip autogenerated files for mobile and gRPC.
+  # Skip autogenerated files for mobile and gRPC as well as copied code for
+  # internal use.
   skip-files:
     - "mobile\\/.*generated\\.go"
     - "\\.pb\\.go$"
     - "\\.pb\\.gw\\.go$"
+    - "internal\\/musig2v040"
 
   skip-dirs:
     - channeldb/migration_01_to_11